The present invention principally relates to negative pressure boosters in which a brake master cylinder of an automobiles is boosted and operated under negative pressure, and more particularly to a booster comprising a booster piston which divides the interior of a booster shell into a first operating chamber internally communicated with a negative pressure source and a second operating chamber, and a valve cylinder protruded at the rear of the booster piston and having a first annular valve seat disposed at the front portion thereof, the valve cylinder slidably receiving a valve piston having a second annular valve seat encircled by said first valve seat and connected to an input rod and including a cylindrical resilient valve body having a valve portion biased to be seated on said first and second valve seats and inside of which is open to the atmosphere, whereby when said valve piston is retracted, said first and second operating chambers are brought into communication through a clearance formed between said first valve seat and said valve portion whereas when said valve piston is moved forward, said second operating chamber and said inside of said valve body are brought into communication through a clearance formed between said second valve seat and said valve portion.
In the booster of the type as described, in an attempt to minimize lost stroke of the input rod in the early stage of operation to prevent the operation delay of the booster piston, it is necessary to reduce the clearance between said valve seat and said valve portion as narrow as possible in an inoperative condition of the input rod, and in an attempt to improve the return responsiveness of the booster piston when the input rod is retracted, it is necessary to increase the clearance between said first valve seat and said valve portion as wide as possible conversely to the former. Thus, there are two requirements contrary to each other.
It is therefore an object of the present invention to provide a booster which can satisfy these two requirements, which can absorb the impact force of retraction of the valve piston and which is quiet and excellent in durability.